The so-called thermic methods represent a significant group of secondary and tertiary methods of oil expoitation, in the course of which the highest possible part of oil reserves which could not be exploited traditionally are attempted to be mobilized partly by oxidation in situ, partly by injecting steam or vapor mixtures.
In the specification of the present invention, the scope is restricted to the injection of steam, a mixture of vapor, steam containing carbon dioxide and mixtures of vapors containing carbon dioxide; within this far-reaching field which includes a plurality of technical problems, we are particularly dealing with the behavior of the clay minerals of reservoir rocks, as they play a most determinant role in defining the hydrodynamic state of reservoir rocks.
It is a well known fact that clay minerals with a layer structure of 2:1, e.g. the montmorillonites, are capable of considerable hydration, as a consequence of which the size of the cells increases in the direction of the C-axis, their stability decreases, and their dispersion begins. In the course of clay dispersion, the content of the suspended material of fluids (in particular water and oil) flowing in the interconnected pore system of the rock increases, which accumulates in certain parts of the rock (due to mechanical filtration or for colloid-chemical reasons) and forms local flow barriers, thus deteriorating the average permeability and increasing detrimental heterogeneity. Swelling and dispersion are especially harmful, if the ionic character of the rock determined by the original fluids, i.e., the natural state of equilibrium, is disturbed by relatively drastic methods, such as steam injection.
It is quite obvious, that not only the state of the clay minerals but also the thermodynamic conditions of the whole environment--in particular, in zones surrounding the well--are changed, while these changes exert a considerable effect, generally disadvantageous changes on the hydrodynamic properties of the porous system, as a consequence of the modified permeability of the rock, as described hereinabove.
If steam or steam also containing organic compounds is injected, it is obvious that a more or less superheated steam phase has to be injected. However, in this way condensation within the rock, i.e., quick and considerable dilution of the aqueous phase cannot be avoided either, so the original ionic character diminishes. As water molecules can be incorporated easily into the clay minerals, with a high energy, the natural ion content of the clays is exchanged, the clays begin to swell and become dispersed; this phenomenon can result--in particular with rocks with a high clay content--in the collapse of the structure, up to the collapse of the zones in the environment of wells.
Summing up what has been said, in addition to other factors, inhibition, elimination of changes in clay minerals, the so-called clay-effect is of utmost importance.